1. Technical Field
The present disclosure relates to electrosurgical instruments and, more particularly, an over-shoe for use in cooperation with electrosurgical instruments for controlling the amount of electrosurgical energy delivered to the tissue.
2. Description of Related Art
A hemostat and/or forceps is a simple plier-like tool which uses mechanical action between its jaws to constrict vessels and is commonly used in open surgical procedures to grasp, dissect and/or clamp tissue. Electrosurgical hemostats and/or forceps utilize both mechanical clamping action and electrical energy to effect hemostasis by heating the tissue and blood vessels, between its jaws, to cut, blend and/or coagulate tissue. An electrode operatively associated with each opposing jaw member is charged to a different electric potential such that when the jaw members grasp tissue therebetween, electrical energy can be selectively transferred through the tissue. A surgeon can cut, blend, coagulate/desiccate and/or simply reduce or slow bleeding, by controlling the intensity, frequency and duration of the electrosurgical energy applied between the electrodes and through the tissue.
“Tissue heating” is generally proportional to the square of the amount of current being generated through tissue while “tissue vaporization” is generally proportional to current and is generally proportional to the amount of energy in an arc. The amount of energy in the arc, in combination with the “Cathode Fall Voltage”, derives the “power for vaporization”. “Thermal spread” is dependent on the amount of heat generated within the tissue and is dependent on tissue resistivity and the arc energy squared. As can be appreciated, the control of “thermal spread” is an important factor in determining and controlling the depth of tissue treatment.
Accordingly, during electrosurgery, an increase or decrease in the amount of current provides and/or creates a different effect on the tissue. This phenomenon is due to a variable referred to as the crest factor (CF). The crest factor can be calculated using the formula: CF=VPEAK1/VRMS, where VPEAK is the positive peak of the waveform and VRMS is the “Root Mean Square” or RMS value of the waveform. The crest factor can also be calculated using the formula: CF=[(1−D)/D]1/2, where D is the duty cycle of the waveform and is defined as D=T1/(T1+T2).
An increase in the crest factor results in more current per arc at a given power setting. Further, “since tissue heating” is proportional to the current through the tissue squared, and “tissue vaporization” is proportional to the amount of current being generated through the tissue, a doubling of current per arc results in four times as much tissue heating and twice the amount of tissue vaporization when an electrode of an electrosurgical hemostat and/or forceps, connected to the electrosurgical generator system, contacts the tissue.
Based on the above formulas, it is evident that when operating an electrosurgical generator system in either the “cut”, “blend”, “coagulate” or seal mode, the range of the crest factor varies from one mode to another. For the purposes herein, “coagulation” is defined as a process of desiccating tissue wherein the tissue cells are ruptured and dried. “Vessel sealing” is defined as the process of liquefying the collagen in the tissue so that it reforms into a fused mass with significantly-reduced demarcation between the opposing tissue structures (opposing walls of the lumen). Coagulation of small vessels is usually sufficient to permanently close them. Larger vessels need to be sealed to assure permanent closure.
Commonly assigned U.S. Application Serial Nos. PCT Application Serial No. PCT/US01/11340; U.S. application Ser. No. 10/116,824; and PCT Application Serial No. PCT/US01/11420 (all of which are hereby incorporated by reference herein) teach that to effectively seal tissue or vessels, especially large vessels, two predominant mechanical parameters must be accurately controlled: 1) the pressure applied to the vessel; and 2) the gap distance between the conductive tissue contacting surfaces (electrodes). As can be appreciated, both of these parameters are affected by the thickness of the vessel or tissue being sealed. Accurate application of pressure is important for several reasons: 1) to oppose the walls of the vessel; 2) to reduce the tissue impedance to a low enough value that allows enough electrosurgical energy through the tissue; 3) to overcome the forces of expansion during tissue heating; and 4) to contribute to the end tissue thickness which is an indication of a good seal. It has been determined that a typical sealed vessel wall is optimum between about 0.001 inches and about 0.006 inches.
With respect to electrosurgically treating relatively smaller vessels, for effective sealing, the pressure applied to the vessel becomes less relevant and the gap distance between the electrically conductive surfaces of the electrodes becomes more significant. In other words, the chances of the two electrodes touching one another during activation increases as the tissue thickness and the vessels become smaller.
As can be appreciated, when cutting, blending or coagulating vessels, the tissue disposed between the two opposing jaw members is essentially destroyed. Other known electrosurgical instruments include blade members or shearing members which simply cut tissue in a mechanical and/or electromechanical manner and, as such, also destroy tissue viability. With respect to electrosurgically treating relatively larger vessels and/or soft tissues (e.g., lung, intestine, lymph ducts, etc.), to promote healing, the above-identified surgical treatments are generally impractical due to the fact that in each instance the tissue or a significant portion thereof is essentially destroyed to create the desired surgical effect (e.g., cutting, blending and/or cauterizing) which does not promote healing.
Accordingly, the need exists for electrosurgical accessories and/or devices (e.g., an electrically conductive or insulative over shoe) which can be used in cooperation with existing electrosurgical instruments (e.g., electrosurgical forceps) for controlling and/or limiting the current (or current per arc on a micro level) and for controlling the degree of tissue heating and the degree of tissue vaporization.
In addition, the need exists for electrosurgical accessories and/or devices (e.g., an electrically conductive or insulative over shoe) for cooperative use with existing electrosurgical instruments (e.g., electrosurgical forceps) which allow for the effective treatment of tissue and the effective maintenance of tissue viability across the treatment area to promote tissue healing.